Spectral properties-based system and method for feeding masterbatches into a plastic processing machine

ABSTRACT

Disclosed is a system for optimizing a match between the color of an in-line part manufactured by a plastic product production machine and the color of a reference part by adjusting the concentration of masterbatch in the mixture of raw material fed to the plastic product production machine. The optimization of the color is based on interlaced spectra of the in-line part and reference part obtained using the same spectrometer, thereby eliminating the requirement for high accuracy spectrometer calibration and allowing a controller of the system to determine the rates at which the base masterbatches are added to the raw material in real time on the manufacturing floor while the plastic product production machine is being operated to manufacture in-line parts.

FIELD OF THE INVENTION

The present invention relates to the dispensing of additive materialinto plastic processing machines in the plastics industry. Inparticular, the present invention discloses a system for optimizing theamount of fed color additive materials (color masterbatches) by in-linemeasurement of the molded product spectral properties, comparing them toa reference material and controlling the feeding device of thedispensing system by using the signal obtained from the comparison andthe processing of spectral properties.

BACKGROUND OF INVENTION

In the modern world, plastics are the material of choice for themanufacture of a seemingly unlimited number of products. These productsare produced by a variety of industrial processes, e.g. injectionmolding, blow molding, extrusion, and 3-D printers. The raw materialthat is fed into the machines used to produce the final products is amixture consisting of: polymers (called resin or virgin in the industry)in the form of small beads, colorants and other additives, e.g. UVinhibitors. The colorants and other additives are supplied asmasterbatches, which are concentrated mixtures of pigments and/oradditives encapsulated during a heat process into a carrier resin whichis then cooled and cut into a granular shape.

Herein the term “masterbatch” is used to refer to a masterbatch thatcontains pigment, i.e. color masterbatch, and the term “base material”is used to refer to polymers or mixtures of polymers.

Herein the term “screw” is used to refer to a screw, dosing mechanism,auger, belt conveyer, or vibratory mechanism of the dispensing system

In order to dispense the required amount of the additivesmaterial—mainly color masterbatch—to be mixed with the base materialvolumetric or gravimetric feeders are commonly utilized. One or moresuch feeders are installed on the throat of the plastic processingmachine.

The volumetric system releases a pre-defined volume ofadditive/masterbatch into the mixing machine. The advantage of thissystem is implementation simplicity by using a feeding screw, where thereleased volume is calibrated to the screw rotation speed. This methodcompromises accuracy for simplicity, since the exact weight (calculatedto be volume multiplied by density) of the released masterbatch materialfor the same rotation speed varies with the masterbatch density, granulesize and other parameters.

U.S. Pat. Nos. 5,103,401, 6,688,493B2 and 6,966,456B2 describegravimetric methods. The gravimetric methods add a weighing mechanismwith a control system to the feeding screw, and then, periodically theexact weight of the released material is measured. The differencebetween the actual weight and the set point is used as the error signalfor the control electronics. The gravimetric method has much greateraccuracy compared to the volumetric method, resulting in saving ofmasterbatch material. A gravimetric system allows the material to bereleased exactly in the amount defined by the set point, usually definedin mass per time unit or percent of the base material. A prior artgravimetric system is shown schematically in FIG. 1.

In both the volumetric and gravimetric cases the masterbatch materialset point is defined empirically and no actual measurements of theproperties of the mixture are made in-line to confirm/adjust it.

Precision color measurement based on optical spectrum is an extremelychallenging process, since fractions of percent of calibration accuracyare required in order to achieve color accuracy better than the colorresolution of the human eye.

It is therefore an object of the present invention to provide a systemfor adjusting and controlling the masterbatch release rate according toan in-line measurement of spectral properties of a product to fit apre-defined spectral signature of a given reference sample.

SUMMARY OF THE INVENTION

In a first aspect the invention is a system for optimizing a matchbetween the color of an in-line part manufactured by a plastic productproduction machine and the color of a reference part by adjusting theconcentration of masterbatch in a mixture of raw material fed to theplastic product production machine. The system comprises:

-   -   a) two illumination light sources;    -   b) two measuring heads;    -   c) one spectrometer;    -   d) a spectrum processing and illumination control unit;    -   e) at least one mechanism configured to add masterbatch to the        mixture of raw material fed to the plastic product production        machine; and    -   f) at least one controller configured to control the rate at        which the mechanism adds masterbatch to the mixture.

The spectrum processing and illumination control unit is configured to:

-   -   i) activate the spectrometer;

ii) activate the two illumination light sources and the two measuringheads intermittently to enable interlaced measurement of the referencesample spectra and the in-line part spectra;

-   -   iii) measure the spectrum of the in-line part received from the        spectrometer;    -   iv) measure the spectrum of the reference part received from the        spectrometer;    -   v) determine the color coordinates of the in-line part and the        reference part from the measured spectra;    -   vi) determine the color coordinates of a set point, which        corresponds to the lowest concentration of masterbatch required        to make the color of the in-line part indistinguishable to the        human eye from the color of the reference part;    -   vii) determine a distance ΔE between the color coordinates of        the in-line part and the color coordinates of the set point;    -   viii) determine a signal for controlling the feed speed of the        mechanism that adds masterbatch to the mixture of raw material        fed to the plastic product production machine.

The controller is configured to control the feed speed of the mechanismthat adds masterbatch to the mixture of raw material fed to the plasticproduct production machine by means of the signal.

The interlaced spectra of the in-line part and the reference part aremeasured simultaneously using the same spectrometer, thereby eliminatingthe requirement for high accuracy spectrometer calibration and allowingthe spectrum processing and illumination control unit to determine thesignal for controlling the feed speed of the mechanism that addsmasterbatch to the mixture of raw material fed to the plastic productproduction machine in real time on a manufacturing floor while theplastic product production machine is being operated to manufacturein-line parts.

In embodiments of the system the set point determined by the spectrumprocessing and illumination control unit is the lowest saturation pointon the MacAdam ellipse around the reference material sample color. Inthese embodiments the spectrum processing and illumination control unitdetermines the set point in one of the following ways:

-   -   a. by maximizing the distance from boundaries of a chromaticity        diagram;    -   b. by minimizing the distance from color coordinates of the        in-line part to a white center point; and    -   c. by mathematical definition of saturation (S) value by        transformation from xyY color space into the HSV color space.

In embodiments of the system the spectrum processing and illuminationcontrol unit determines the set point by means of an iterative process.

In embodiments of the system the spectrum processing and illuminationcontrol unit determines the distance ΔE between the in-line color partand the set point on a chromaticity diagram using the CIEDE2000 formula.

In embodiments of the system the signal for controlling the feed speedof the mechanism that adds masterbatch to the mixture of raw materialfed to the plastic product production machine is defined asErr=ΔE*ƒ(S ₀ −S),where S₀ and S are saturation values of the reference sample and thein-line part colors respectively, and ƒ(S₀−S)=f(x) is a weightingfunction.

In embodiments of the system the spectrum processing and illuminationcontrol unit determines the color of the in-line part from thecombination of three base masterbatches and determines the signals forcontrolling the feed speeds of the mechanisms that adds masterbatch tothe mixture of raw material fed to the plastic product productionmachine by projecting the ΔE vector on the axes defined by vectorsconnecting the locations of the base masterbatches on the chromaticitydiagram.

In embodiments of the system the measurement head comprises:

-   -   a) a first facet of an illumination optical fiber configured to        conduct light to the sample from an illumination source, wherein        the light enters the illumination optical fiber at a second        facet; and    -   b) a first facet of a collection optical fiber configured to        conduct light reflected from or transmitted through the sample        to a spectrometer, wherein the light exits the collection        optical fiber at a second facet.

In these embodiments the measurement head is characterized in that itcomprises two crossed polarizers, wherein:

-   -   i) a first of the crossed polarizers is located at the first        facet of the illumination optical fiber;    -   ii) a second of the crossed polarizers is located at the first        facet of the collection optical fiber;    -   iii) when light from the illumination source is reflected from        the sample, each of the first and the second crossed polarizers        is located near the focal point of a one or a plurality of        aspheric or spherical lenses configured to collimate and direct        light emitted from the first facet of the illumination optical        fiber onto the sample and to direct light reflected from the        sample into the first facet of the collection optical fiber; and    -   iv) when light from the illumination source is transmitted        through the sample, each of the first and the second crossed        polarizers is respectively located near the focal point of one        of two or a plurality of aspheric or spherical lenses, wherein a        first of the two or a plurality of lenses is configured to        collimate light and direct light emitted from the first facet of        the illumination optical fiber onto the sample and a second of        the two or a plurality of lenses is configured to collect and        direct light transmitted through the sample into the first facet        of the collection optical fiber.

In these embodiments the distances between the polarizers and the focalpoint of the lenses is chosen such that the change of the measuredsignal with the distance between the probe and the sample is minimized.

In a second aspect the invention is a measurement head configured formeasuring the spectrum of a sample. The measurement head comprises:

-   -   a) a first facet of an illumination optical fiber configured to        conduct light to the sample from an illumination source, wherein        the light enters the illumination optical fiber at a second        facet; and    -   b) a first facet of a collection optical fiber configured to        conduct light reflected from or transmitted through the sample        to a spectrometer, wherein the light exits the collection        optical fiber at a second facet.

The measurement head is characterized in that it comprises two crossedpolarizers, wherein:

-   -   i) a first of the crossed polarizers is located at the first        facet of the illumination optical fiber;    -   ii) a second of the crossed polarizers is located at the first        facet of the collection optical fiber;    -   iii) when light from the illumination source is reflected from        the sample, each of the first and the second crossed polarizers        is located near the focal point of a one or a plurality of        aspheric or spherical lenses configured to collimate and direct        light emitted from the first facet of the illumination optical        fiber onto the sample and to direct light reflected from the        sample into the first facet of the collection optical fiber; and    -   iv) when light from the illumination source is transmitted        through the sample, each of the first and the second crossed        polarizers is respectively located near the focal point of one        of two or a plurality of aspheric or spherical lenses, wherein a        first of the two or a plurality of lenses is configured to        collimate light and direct light emitted from the first facet of        the illumination optical fiber onto the sample and a second of        the two or a plurality of lenses is configured to collect and        direct light transmitted through the sample into the first facet        of the collection optical fiber.

In the measurement head the distances between the polarizers and thefocal point of the lenses is chosen such that the change of the measuredsignal with the distance between the probe and the sample is minimized.

All the above and other characteristics and advantages of the inventionwill be further understood through the following illustrative andnon-limitative description of embodiments thereof, with reference to theappended drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 schematically shows a gravimetric additive feeder system for aninjection molding machine according to the prior art;

FIG. 2(a) schematically shows a calculation method of the reference andthe part color coordinates from the spectrum data according to anembodiment of the invention;

FIGS. 2(b)-2(c) schematically shows a derivation of the optimal errorsignal for the screw control loop on the chromaticity diagram, based onminimum saturation requirement, corresponding to the minimum consumptionof the color masterbatch, lying within MacAdam ellipse ofundistinguishable colors, according to an embodiment of the invention;

FIG. 2(d) schematically shows an additional algorithm for derivation ofthe optimal error signal for the screw control loop on the chromaticitydiagram, based directly on the minimum consumption of the colormasterbatch, lying within MacAdam ellipse of undistinguishable colors,according to an embodiment of the invention;

FIG. 3 shows a schematic layout of the comparative spectral measurementbased masterbatch feeding screw control system concept according to anembodiment of the invention;

FIG. 4(a) schematically shows an external lighting optical fiber basedimplementation example of the comparative spectral measurementmasterbatch feeding screw control system, according to an embodiment ofthe invention;

FIG. 4(b) schematically shows an alternative implementation of thecomparative spectral measurement masterbatch feeding screw controlsystem according to an embodiment of the invention, when the referencesample is periodically measured with the same spectrometer as thein-line manufactured parts;

FIG. 4(c) schematically shows an external lighting optical fiber basedimplementation example of the comparative spectral measurement systemwith multiple measurement heads, according to an embodiment of theinvention;

FIG. 5 schematically shows a fiber delivered lighting optical fiberbased implementation example of the comparative spectral measurementmasterbatch feeding screw control system, according to an embodiment ofthe invention;

FIG. 6 schematically shows the system of the invention according to anembodiment of the invention which is intended for mixing multiple colormasterbatches;

FIGS. 7(a)-7(b) schematically show an example of the method of theinvention according to an embodiment of the invention to control themixing quantities of the masterbatches;

FIG. 8 schematically shows a layout of the illumination module of theinvention with reduced effect of specular reflections, used for themeasurement head of the comparative spectral measurement basedmasterbatch feeding screw control system;

FIG. 9 schematically shows a layout of the system of the inventionaccording to an embodiment of the present invention where the referencepart is substituted by interchangeable reference samples array withknown spectral properties, used for automatic absolute calibration ofthe spectrometer system;

FIG. 10 schematically shows an embodiment of spectrometer 36 that can beused to carry out the invention;

FIGS. 11(a) and 11(b) schematically show in more detail an example of anoptical probe assembly comprising two measurement heads such as shown inFIG. 5;

FIGS. 12(a) and 12(b) schematically show in more detail an example of anoptical probe assembly comprising multiple measurement heads such asshown in FIG. 4(c);

FIG. 13 schematically shows an embodiment of a setup for introducing asingle light source into a plurality of optical fibers while only onefiber at a time is illuminated;

FIG. 14(a) schematically shows an embodiment of an optical layout for ameasurement head for use in reflective measurements;

FIG. 14(b) schematically shows an embodiment of an optical layout for ameasurement head for use in transmissive measurements; and

FIG. 14(c) shows an example of the dependence of the color differencebetween the reference and the sample parts on the distance from the lensof the measurement head to the sample part surface.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a system for optimizing the amount of fed colormasterbatches in a plastic product production line by in-linemeasurement of the spectral properties of the product and a referenceobject, processing of spectral properties of the product and referenceobject, comparing the processed spectral properties, and controllingfeeding screws by using the signals obtained from the. The inventionuses a spectrometer based system with robust calibration-freedifferential measurement of the manufactured part and the referencepart.

FIG. 1 schematically shows a prior art gravimetric additive mixingsystem. The feed screw 11 meters masterbatch or another additive intothe main flow of material. The masterbatch is drawn from supplycontainer 12 into hopper 13 where it is weighed with a loss-in-weightbalance and distributed in the flow of base material out of hopper 14.Metering of the masterbatch is synchronized with the molding machine'sfeed screw 15.

Since it is impossible in most of the cases to weigh a discrete portionof the additive that is fed during a cycle time due to its tiny weightand the very noisy and shaky environment of the production area, thesystem uses a closed loop feedback operation to control the weight ofthe portion by weighing a number of dispensed portions usingloss-in-weight of the hopper, dividing the weight by the number ofportions and, controlling the speed of the screw feeder motor todispense in a given time portions each with a predetermined weight for agiven interval of time.

FIG. 2(a) schematically shows a method for optimal control of thefeeding screw speed that is used by the controller of the system of thepresent invention. In the first step 201 an optical spectrum is measuredfor a reference material part and for an in-line part. FIG. 2(b) showsthe optical spectrum of the reference part 210 and of the in-line part220 and the color response functions. In the second step 202 the colorcoordinates of the in-line part (x, y, Y) and of the reference material(x0, y0, Y0) in xyY color space are determined as follows:X=∫ ₃₈₀ ⁷⁸⁰ I(λ) x (λ)dλY=∫ ₃₈₀ ⁷⁸⁰ I(λ) y (λ)dλZ=∫ ₃₈₀ ⁷⁸⁰ I(λ) z (λ)sλwhere I(λ) is the spectral power density of the measured sample. Theobtained coordinates are translated into the CIE xyY color space whichcan be seen in FIG. 2(c), by well-known linear transformation. Where x,y, z, are standard observer color matching functions and thetransformation from XYZ to xyY is:

$x = \frac{X}{X + Y + Z}$ $y = \frac{Y}{X + Y + Z}$ Y = Y

FIG. 2(c) shows the chromaticity diagram. The “star” point 228 is thecolor coordinates of the masterbatch material. Point 225 and point 226are the color coordinates received in step 202 of the in-line part andthe reference part respectively. Star 223 in the center of thechromaticity diagram is the white color, which is the lowest saturationvalue point. As a first approximation, varying the masterbatchconcentration will move the color coordinate of the in-line part alongthe dashed line. The maximum saturation value colors are located on theboundaries. To achieve higher saturation value, more color masterbatchshould be added to the base material.

A human eye cannot differentiate colors within a certain area 224,called the MacAdam ellipse, surrounding a point on the chromaticitydiagram. The size of the ellipse varies with the location of the pointon the chromaticity diagram. Boundaries of the undistinguishable colorsregion are defined by the CIEDE2000 standard.

In step 203 a set point 227 is determined. The set point 227 is thelowest saturation point on the undistinguishable color boundary, i.e.the MacAdam ellipse, around the reference material sample color 226(x0,y0,Y0). The saturation point corresponds to the lowest concentrationof masterbatch required to make the color of the in-line partindistinguishable to the human eye from the color of the reference part.The lowest saturation point can be found by maximizing the distance fromthe boundaries of the chromaticity diagram or by minimizing the distancefrom the color coordinates of the in-line part to the white center point223 or by mathematical definition of saturation (S) value bytransformation from xyY color space into the HSV color space.

In step 204, using the CIEDE2000 formula the distance ΔE between thein-line part color 225 and the above defined set point 227 isdetermined. In step 205 the signal used for controlling the feedingscrew rotation speed is calculated. This signal is defined as:Err=ΔE*ƒ(S ₀ −S),where S₀ and S are saturation values of the reference sample and thein-line part colors respectively, and ƒ(S₀−S)=f(x) is a weightingfunction, which, for example, can take the values: f(x)=−1, if x<−1;f(x)=1, if x>1 and f(x)=x otherwise.

In the last step 206, the error signal is used to control the feedingscrew rotation speed.

FIG. 2(d) provides a refined method for determination of the optimalfeeding screw error input, based on the fact that the color resultingfrom various pigment (masterbatch) concentrations does not follow astraight line but follows a curved path 238 as shown in FIG. 2(c). Themore exact behavior is described by a well know Kubelka Munk model (seefor example Georg A. Klein, “Industrial Color Physics”, Springer 2010,pg. 326-337). In this case the initial error is evaluated following thestraight line between the color coordinates of the in-line part 225 andthose of the reference part 226 resulting in a correction ΔE₁ in thesame manner as described in the FIG. 2(c). The color of the in-line partresulting from the masterbatch concentration adjusted using ΔE₁ will notlie on the MacAdam ellipse 224 but will have coordinates 229. From thispoint, the error is evaluated again using a straight line between points229 and 224 and the process is iterated, until the in-line part colorcoordinate crosses the MacAdam ellipse 224 (or its approximation by someconstant ΔE value, typically about 2.5) from the low saturation side.The actual end result of this method is that the resulting in-line partcolor coordinate is 210 instead of 227 as is expected from the simplermodel of FIG. 2(c).

FIG. 3 shows a system according to an embodiment of the presentinvention. A schematic layout of the comparative spectral measurementbased masterbatch feeding screw control system is shown. Precision colormeasurement based on optical spectrum is an extremely challengingprocess, since fractions of percent of calibration accuracy arerequired. Due to this fact, spectrometers are rarely used onmanufacturing floors, but rather in analytic and quality controllaboratories, since maintaining those high accuracy calibrations israther impractical on manufacturing floors. The method which isdescribed above in FIG. 2 is implemented by measuring the referencesample and the in-line part spectrum simultaneously using the samespectrometer. In this case deviations from nominal spectrometercalibration are the same for both measurements of the in-line part andthe reference part; therefore no high accuracy spectrometer calibrationis required. Due to a control feedback loop used for the feeding screwcontrol, color difference error inaccuracy resulting from thespectrometer calibration deviation turns out to be insignificant.

The schematic layout of the differential spectrometer, which measuresthe in-line part and the reference part while comparing their colorcoordinates, is shown in FIG. 3). The optical signals reflected backfrom the measurement heads for the reference material part 31 and thein-line sample 32 are combined by 50%/50% beamsplitter 35 and sent to aspectrometer 36. Each measurement head utilizes a white light source 33(implemented by LED, halogen lamp, fluorescent lamp, incandescentsource, supercontinuum laser or any other wide spectrum light source). Acosine corrector or homogenizer 34 is used on the entrance into thelight collection optics in order to minimize the spectrum dependence onthe measurement geometry. The light sources 33 for the reference andin-line measurement heads are operated intermittently while enablinginterlaced measurement of the reference sample and the in-line partspectra. Both spectra are analyzed in the spectrum processing andillumination control unit 37, according to the method of the presentinvention as described above and the resulting value of the “errorsignal” 39 is sent to the feeding screw rotation velocity controller 38.

FIG. 4(a) schematically shows an example of another implementation ofthe system of the present invention according to an embodiment of theinvention, wherein the light is collected from both measurement headsfor both samples 31, 32 using optical fibers 41 and 42, combined by 2×1fiber combiner 43 and conducted through optical fiber 44 to thespectrometer 36.

FIG. 4(b) schematically shows an alternative implementation of thesystem using a single spectral measurement head for both the referencepart 31 and the in-line part 32. A mechanism, symbolically shown bydouble headed arrow 45, periodically moves the in-line part 32 to theside and moves the reference part 31 under the measurement head tomeasure its color spectrum. The parameters of the color of the referencepart are stored in the memory and used for calculating the feeding screwcontrol error in the manner as disclosed in FIG. 2.

FIG. 4(c) schematically shows an example of an implementation of thesystem according to an embodiment of the invention. According to thisembodiment of the invention the system utilizes multiple (more than 2)measurement heads for measuring the in-line part samples 32 ₁, 32 ₂ . .. 32 _(n-1) at different locations in order to evaluate the colorhomogeneity for quality assurance. In this embodiment the feeding screwcontrol 38 uses the average signal of all measurement heads, theirscatter and their color deviation from the reference part 31. N×1 fibercombiner 43 is utilized to combine the multiple n−1 measurement headsinto a single fiber attached to a spectrometer 36.

FIG. 5 schematically shows an implementation of a system according toanother embodiment of the invention, wherein a single light source 33used. The light from source 33 passes through fiber branch 57 until itis split into two equal parts by 2×2 optical fiber splitter/combiner 55from which light is transmitted to both measurement heads through thefiber branches 58 and 59. The return signal from both measurement headsare transmitted through the same fiber branches 58 and 59 and arecombined by the same 2×2 splitter/combiner 55 and sent to thespectrometer via the fiber branch 56. Alternatively, the light source 33may comprise a plurality of separate light sources with the same ordifferent properties like spectral intensity distribution and power, allcombined with a beam combiner into a single fiber 57. This way aspecific required spectral distribution may be achieved, for example amore balanced intensity distribution spectrum may be achieved bycombining a halogen lamp, lacking intensity at short wavelengths, with ablue or white light emitting diode.

In the embodiment shown in FIG. 5, separate measurements of thereference and of the in-line parts is achieved by the following method.The light source operates continuously. The reference sample 31 isalways in place and its spectrum is measured by the spectrometer 36 whenthere is no in-line part near the measurement head. Once a discretesignal 50 from the in-line parts measurement head indicates that anin-line part moving on the production line (symbolically shown by arrow51) is in place under the measurement head, a combined signal from thein-line part and the reference part is measured. The spectrum of thein-line part is obtained by subtracting the spectrum of the referencepart from the combined signal. The inherent advantage of this embodimentof the method of the invention compared to separate intermittentillumination is that precisely the same illumination is used formeasuring both the reference and the in-line parts, improving the resultaccuracy. However, this embodiment causes a 50% loss for the spectrumsignal compared to the implementation shown in FIG. 4.

FIG. 6 schematically shows the layout of the system of the presentinvention which is intended for mixing multiple color masterbatches(called the “base” masterbatches) in order to obtain a mixture in whichthe color of the in-line part is coincident with that of the referencepart after the masterbatches are added into the processing machine. Thesystem comprises a separate feeding module 61 ₁, 61 ₂, . . . 61 _(n)that can be either volumetric, gravimetric or any other quantificationmethod based (the gravimetric example is shown in FIG. 1) for each ofthe base masterbatches. The feeding mechanisms of those modules arecontrolled by a differential spectrometer system 62 that processessignal from measurement heads 63 and 64 for the reference part and thein-line parts respectively as disclosed with respect to the previousFIGS. 3-5, wherein each module is controlled by a different controller38 ₁, 38 ₂, . . . 38 _(n) using the method that is described hereinbelow in FIGS. 7(a)-(b).

FIGS. 7(a)-(b) show an example of a method according to an embodiment ofthe invention, which is used to control the mixing quantities of themasterbatches. The goal of the method is to define the set point 227,which is located within the MacAdam ellipse 224 of undistinguishablecolors surrounding the coordinates 226 of the reference part, at thelowest color saturation point. This ensures that the in-line part coloris indistinguishable from the reference sample and the manufacturingprocess consumes the least amount of the masterbatch material. Theoutput of the algorithm is the amount of increase/decrease of thepercent of each particular base masterbatch in the mixture ofmasterbatches.

First, as can be seen in FIG. 7(a) the MacAdam ellipse 224 and therelative error ΔE 72 are calculated in the same way described above inFIG. 2. The stars 73, 74 and 75 are the color coordinates of eachdifferent base masterbatch respectively. Open circle 225 and filledcircle 225 are the color coordinates of the in-line and reference partsrespectively. In FIG. 7(b) the ΔE vector 72 is projected on the axesdefined by vectors connected the base masterbatches. The projections areused as the error corrections 39 ₁, 39 ₂ . . . 39 _(n) of the loopscontrolling the feeding screw (using a standard PI or PID control loop,as commonly done in all industrial control systems), intended tominimize the differences between the colors of the reference part andthe in-line parts.

Practically, three base masterbatches are enough to span most of thecolors lying within the triangle connecting them. Alternatively, morebase masterbatches can be used so that more than one possibility existsto determine the component of the error vector ΔE for each of the basemasterbatches. In this case a predefined merit function (for examplecost or amount of added material) is used to select the optimalcombination of masterbatches to minimize the error between the referencesample and the manufactured parts.

FIG. 8 shows the schematic layout of an embodiment of an illuminationand light collection module, i.e. a measurement head. Typically, thelighting conditions affect greatly the resulting color coordinates ofthe measured sample. To minimize the influence of random light on themeasurements, in embodiments of the invention light baffles and opticalelements are utilized to accurately control the lighting conditions inorder to minimize the effect of specular reflections by accurate opticaldesign of the illumination and light collection means.

In the embodiment shown in FIG. 8 light from the illumination source 33passes through a polarizer 81. The light reflected from the part 32 iscollected by the optical fiber 82. Another polarizer 83 with orientationperpendicular to that of polarizer 81 is introduced in front of thefiber 82. The polarizer 83 blocks most of the specular reflections andallows only diffusely scattered light to enter the collection fibersince diffuse reflections are mostly un-polarized, while specularreflection mostly maintains the incident light polarization. In order tofurther eliminate stray light, a baffle 84 is introduced around thefiber in order to block the stray light 85.

In other embodiments specular reflected light can be eliminated by othercombinations of polarizers such as a half wave plate with linearpolarizer, which is known to prevent light from being reflectedbackwards and circular polarizers.

FIG. 9 schematically shows a layout for an embodiment of a method forautomatic absolute calibration of the differential spectrometer.Usually, in cases where there is no reference sample available, and thepart color is defined by color coordinates (for example xyY, Lab, Luv,HSV, sRGB, XYZ or others) there is a need to define the reference point226 in FIG. 2(c) numerically. In order to do so, the measurement of thecolor of the manufactured parts should be calibrated to absolute colorcoordinates. However, in the system of the present invention, theabsolute color coordinates are parameters that are not required whileusing differential measurement as disclosed herein. The presentinvention discloses a method for automatic on-line calibration of thesystem without operator intervention eliminating the need for accurateperiodic calibrations, which require highly qualified personnel and aresensitive to changes in environmental conditions, vary with time, etc.

As can be seen in FIG. 9, a single reference sample is replaced by anarray 90, which contains a plurality of reference samples 91 havingdistinct known spectral properties. This array 90 of reference samples91 is attached to actuation means 92 capable of moving reference samples91 so that the spectrum of only one of them is measured at a time. Thereference samples can either reflect the light from source 33 as shownin FIG. 9 or transmit the incident light, in which case the referencesample should be located between the illumination means 33 and themeasurement fiber 42.

The calibration procedure is activated periodically, by measuring eachof the reference samples. The present invention system's spectralresponse calculation is performed from the comparison of the measuredspectra with the known one for each sample. One example of such acalculation is to divide the obtained spectrum by the known one for eachsample and averaging the results for a plurality of reference samples.Other more sophisticated and accurate methods for calculation of thesystem response from the plurality of known reference samplesmeasurements are known.

FIG. 10 schematically shows an embodiment of spectrometer 36 that can beused to carry out the invention. The optical layout of this embodimentof spectrometer 36 is based on a well-known Czerny-Turner monochromatorwith the addition of a correction element before the linear sensor arraythat is introduced in order to compensate for aberrations of the opticalelements. Use of this correction element enables low f-number designswithin compact physical dimensions.

In FIG. 10, the collected light is transmitted to the spectrometer 36 byoptical fiber 102. The input fiber light is limited in the horizontaldirection by a vertical slit 104 ranging from 10-500 microns in width,depending on the required resolution. The light passing through slit 104is reflected from a first concave mirror 106 located at a distance equalto its focal length from slit 104 in order to collimate the light fromthe fiber 102. The collimated beam is diffracted by a diffractiongrating 108 having, for example, 300 grooves/mm and focused by a secondconcave mirror 112 onto a sensor array 116 after passing through acorrector element 114. In the simplest case the corrector element 114 isa cylindrical lens that compensates for the strong astigmatism from theangled mirrors. This arrangement allows optical resolution below 10 nmwith 1 mm input optical fiber with a numerical aperture 0.5. In a morecomplex setting allowing higher resolution, more complex correctionelements might be used, e.g. phase masks, diffractive elements, ormultiple optical elements.

FIGS. 11(a) and 11(b) schematically show in more detail an example of anoptical probe assembly comprising two measurement heads such as shown inFIG. 5. FIG. 11(a) is an overall view of the assembly and FIG. 11(b) isa magnified view of section A in FIG. 11(a) showing the internalfeatures of the branches. Light from an illumination source isintroduced into two optical fibers 57 a and 57 b within the illuminationbranch 57. After passing through a 2×2 optical fiber splitter/combiner55 each fiber 57 a and 57 b is further guided by a separate branch 58and 59 to measurement heads 110 and 111 for the in-line part andreference parts respectively. Fibers 112 a and 112 b within branches 58and 59 return light collected by measuring heads 110 and 111respectively and either pass through optical fiber splitter/combiner 55to separate signal branches 56 a and 56 b or are optionally combined bythe optical fiber splitter/combiner 55 into a single signal branch 56 asin FIG. 5.

FIGS. 12(a) and 12(b) schematically show in more detail an example of anoptical probe assembly comprising multiple measurement heads such asshown in FIG. 4(c). FIG. 12(a) is a schematic view of the variousbranches bringing light to and collected light from the measurementheads 110 a-110 n-1 and 111. FIG. 12(b) schematically shows the routingof the illumination fibers 115 and the collection fibers 116 within theoptical probe assembly wherein the collection fibers 116 are combinedinto a single fiber using an n×1 optical fiber combiner 117.

FIG. 13 schematically shows an embodiment of a setup for introducing asingle light source into a plurality of optical fibers while only onefiber at a time is illuminated. The light source 130, which isoptionally followed by an optical system to create a uniform lightdistribution at the plane 131 of the facets 132 of the optical fibers,is filtered by optical filter 133 to remove all unnecessary radiation inorder to decrease the heat load on fiber facets. For color measurementapplications, the filter reflects infrared and transmits visible light.An opaque disk 134 with a plurality of holes is attached to a servomotor 135. The locations of the holes are determined in such a way thatas the disk rotates, a different fiber fact is illuminated, while lightto all the others is blocked. Alternatively, a steadily rotating DCmotor can be used with a disk having tangential slits rather than holes,sequentially exposing each fiber for a predetermined period of time.

FIG. 14(a) schematically shows an embodiment of an optical layout for ameasurement head for use in reflective measurements. In this embodimentthe measurement head 140 is located at a distance h from the frontsurface of sample 141. This design addresses two issues with remotecolor measurement—dependence on the distance to the sample anddependence on the surface angle due to a varying amount of specularreflections entering the collection system. The optical design allowsminimization of both effects. Both illumination fibers 142 and lightcollection fibers 143 are located near the focal point of the asphericor spherical lens 144. Using a 0.25-3 mm diameter optical fiber forillumination and an aberration minimized lens, the light distribution onthe sample surface is such, that the reflected light collected by thecollection fiber is independent of the distance to the sample surfacewithin at least half of the lens focal length. This effect is shown inthe FIG. 14(c), which is a graph showing the dependence of the colordifference between the reference and the sample parts on the distancefrom the lens of the measurement head to the sample part surface. Thesurface angle dependence effect is minimized by introducing two crossedpolarizers (i.e. for example two linear polarizers wherein one isvertically oriented on the other is horizontally oriented as indicatedby arrows in the section view of FIG. 14(a)). The polarizers block bothlight scattered from the lens surface and the specular reflection fromthe sample surface.

FIG. 14(b) schematically shows an embodiment of an optical layout for ameasurement head for use in transmission color measurement, wherein theillumination and collection fibers are located at opposite sides of thesample.

Although embodiments of the invention have been described by way ofillustration, it will be understood that the invention may be carriedout with many variations, modifications, and adaptations, withoutexceeding the scope of the claims.

The invention claimed is:
 1. A system for optimizing a match between thecolor of an in-line part manufactured by a plastic product productionmachine and the color of a reference part by adjusting the concentrationof masterbatch in a mixture of raw material fed to the plastic productproduction machine, the system comprising: a) two illumination lightsources; b) two measuring heads; c) one spectrometer; d) a spectrumprocessing and illumination control unit; e) at least one mechanismconfigured to add masterbatch to the mixture of raw material fed to theplastic product production machine; and f) at least one controllerconfigured to control the rate at which the mechanism adds masterbatchto the mixture; wherein: A) the spectrum processing and illuminationcontrol unit is configured to: i) activate the spectrometer; ii)activate the two illumination light sources and the two measuring headsintermittently to enable interlaced measurement of the reference samplespectra and the in-line part spectra; iii) measure the spectrum of thein-line part received from the spectrometer; iv) measure the spectrum ofthe reference part received from the spectrometer; v) determine thecolor coordinates of the in-line part and the reference part from themeasured spectra; vi) determine the color coordinates of a set point,which corresponds to the lowest concentration of masterbatch required tomake the color of the in-line part indistinguishable to the human eyefrom the color of the reference part; vii) determine a distance ΔEbetween the color coordinates of the in-line part and the colorcoordinates of the set point; viii) determine a signal for controllingthe feed speed of the mechanism that adds masterbatch to the mixture ofraw material fed to the plastic product production machine; and B) thecontroller is configured to control the feed speed of the mechanism thatadds masterbatch to the mixture of raw material fed to the plasticproduct production machine by means of the signal; wherein, measuringinterlaced spectra of the in-line part and the reference part arecarried out simultaneously using the same spectrometer, therebyeliminating the requirement for high accuracy spectrometer calibrationand allowing the spectrum processing and illumination control unit todetermine the signal for controlling the feed speed of the mechanismthat adds masterbatch to the mixture of raw material fed to the plasticproduct production machine in real time on a manufacturing floor whilethe plastic product production machine is being operated to manufacturein-line parts.
 2. The system of claim 1, wherein the set pointdetermined by the spectrum processing and illumination control unit isthe lowest saturation point on the MacAdam ellipse around the referencematerial sample color.
 3. The system of claim 2, wherein the spectrumprocessing and illumination control unit determines the set point in oneof the following ways: a. by maximizing the distance from boundaries ofa chromaticity diagram; b. by minimizing the distance from colorcoordinates of the in-line part to a white center point; and c. bymathematical definition of saturation (S) value by transformation fromxyY color space into the HSV color space.
 4. The system of claim 1,wherein the spectrum processing and illumination control unit determinesthe set point by means of an iterative process.
 5. The system of claim1, wherein the spectrum processing and illumination control unitdetermines the distance ΔE between the in-line color part and the setpoint on a chromaticity diagram using the CIEDE2000 formula.
 6. Thesystem of claim 1, wherein the signal for controlling the feed speed ofthe mechanism that adds masterbatch to the mixture of raw material fedto the plastic product production machine is defined asErr=ΔE*ƒ(S ₀ −S), where S₀ and S are saturation values of the referencesample and the in-line part colors respectively, and ƒ(S₀−S)=f(x) is aweighting function.
 7. The system of claim 1, wherein the spectrumprocessing and illumination control unit determines the color of thein-line part from the combination of three base masterbatches anddetermines the signals for controlling the feed speeds of the mechanismsthat adds masterbatch to the mixture of raw material fed to the plasticproduct production machine by projecting the ΔE vector on the axesdefined by vectors connecting the locations of the base masterbatches onthe chromaticity diagram.
 8. The system of claim 1, wherein themeasurement head comprises: a) a first facet of an illumination opticalfiber configured to conduct light to the sample from an illuminationsource, wherein the light enters the illumination optical fiber at asecond facet; and b) a first facet of a collection optical fiberconfigured to conduct light reflected from or transmitted through thesample to a spectrometer, wherein the light exits the collection opticalfiber at a second facet; the measurement head characterized in that itcomprises two crossed polarizers, wherein: i) a first of the crossedpolarizers is located at the first facet of the illumination opticalfiber; ii) a second of the crossed polarizers is located at the firstfacet of the collection optical fiber; iii) when light from theillumination source is reflected from the sample, each of the first andthe second crossed polarizers is located near the focal point of a oneor a plurality of aspheric or spherical lenses configured to collimateand direct light emitted from the first facet of the illuminationoptical fiber onto the sample and to direct light reflected from thesample into the first facet of the collection optical fiber; and iv)when light from the illumination source is transmitted through thesample, each of the first and the second crossed polarizers isrespectively located near the focal point of one of two or a pluralityof aspheric or spherical lenses, wherein a first of the two or aplurality of lenses is configured to collimate light and direct lightemitted from the first facet of the illumination optical fiber onto thesample and a second of the two or a plurality of lenses is configured tocollect and direct light transmitted through the sample into the firstfacet of the collection optical fiber.
 9. The system of claim 8 whereinthe distances between the polarizers and the focal point of the lensesis chosen such that the change of the measured signal with the distancebetween the probe and the sample is minimized.
 10. A measurement headconfigured for measuring the spectrum of a sample, the measurement headcomprising: a) a first facet of an illumination optical fiber configuredto conduct light to the sample from an illumination source, wherein thelight enters the illumination optical fiber at a second facet; and b) afirst facet of a collection optical fiber configured to conduct lightreflected from or transmitted through the sample to a spectrometer,wherein the light exits the collection optical fiber at a second facet;the measurement head characterized in that it comprises two crossedpolarizers, wherein: i) a first of the crossed polarizers is located atthe first facet of the illumination optical fiber; ii) a second of thecrossed polarizers is located at the first facet of the collectionoptical fiber; iii) when light from the illumination source is reflectedfrom the sample, each of the first and the second crossed polarizers islocated near the focal point of a one or a plurality of aspheric orspherical lenses configured to collimate and direct light emitted fromthe first facet of the illumination optical fiber onto the sample and todirect light reflected from the sample into the first facet of thecollection optical fiber; and iv) when light from the illuminationsource is transmitted through the sample, each of the first and thesecond crossed polarizers is respectively located near the focal pointof one of two or a plurality of aspheric or spherical lenses, wherein afirst of the two or a plurality of lenses is configured to collimatelight and direct light emitted from the first facet of the illuminationoptical fiber onto the sample and a second of the two or a plurality oflenses is configured to collect and direct light transmitted through thesample into the first facet of the collection optical fiber.
 11. Themeasurement head of claim 10 wherein the distances between thepolarizers and the focal point of the lenses is chosen such that thechange of the measured signal with the distance between the probe andthe sample is minimized.